1. Field of the Invention
The present invention pertains to the art of transmission lines for high frequency electronics, and more particularly to the dielectric support structure for the center conductor of a coaxial transmission line.
2. Art Background
Transmission lines for high frequency signal propagation typically consist of two conductors separated by a material that can hold an electric charge (a dielectric). There are two important characteristics of a transmission line: its impedance and maximum operating frequency, both of which are determined by the relative size and spacing of the conductors, and the dielectric constant of the material separating them. Maximum operating frequency is limited by the fact that if the dimensions of the transmission line are greater than a certain fraction of the wavelength that is being propagated, then unwanted modes develop which are detrimental. Therefore, as the operating frequency of the transmission line increases, the characteristic dimensions of the transmission line components must be decreased. Control of line impedance is critical since a portion of the signal is reflected back whenever there is an impedance mismatch. As a result, it is necessary to maintain constant impedance through the entire signal path in order to minimize unwanted reflections.
Coaxial structures are a common form of transmission line with air typically used as the dielectric. Classical analysis shows that the characteristic impedance of a coaxial transmission line is proportional to the logarithm of the inner diameter of the outer conductor to the diameter of the inner conductor. To maintain the center conductor concentrically within the outer conductor a support structure is used, with the center conductor surrounded by a dielectric material. Glass or other ceramics are often used, with a glass-to-metal seal usually used as a support. Since the dielectric constant of the material used to support the center conductor is higher than that of air, something must be changed to maintain a constant impedance through the support structure. Either the diameter of the outer conductor must be increased, or the diameter of the inner conductor decreased to maintain proper impedance. It is more common to decrease the diameter of the inner conductor to prevent unwanted modes from developing as previously discussed. For frequencies in the millimeter-range, 80 GHz and above, the required diameter of the inner conductor is on the order of a few thousandths of an inch when it is decreased to maintain the characteristic impedance, usually on the order of 50 Ohms. As a result, the mechanical strength of the center conductor is severely compromised.
An asymmetrical support structure for a high-frequency transmission line consists of an outer conductor providing a ground plane, a center conductor maintaining a constant diameter coaxially supported above the ground plane by an electrically insulating material forming a dielectric, and electromagnetic absorbing material between the dielectric and the outer conductor in the area away from the ground plane.